How can I stop my Worcester 24i from building up too much pressure ?

Come across this before! The plate to plate domestic hot water heat exchanger (the thing that looks like a large silver sardine can) has developed a hole between the internal plates that separate the heated boiler water from the fresh water on its way to the hot tap!!
This results in the higher pressure mains water over pressurising the boiler water.
A faulty batch of these heat exchangers was produced 7 or 8 years ago.
I'd look at the service manual/you tube to see if you could do this work, and if so get spare from ebay.
Good Luck.

allgasinstaller
December 2016

Possibly the expansion vessel membrane has failed.

You should be able to check. There should be a Schrader type valve (Like on a car tyre) visible on the expansion vessel. Press the little pip in the centre (Like you were letting a car tyre down). If any water comes out, you need a new vessel.

A new boiler specific replacement can be pricey. As a less expensive (and more convenient solution for later maintenance) you could get an aftermarket one fitted externally to the boiler (assuming there is somewhere on the return pipe to fit it put of sight).

Remember to pressurise the vessel correctly in line with the boiler instructions before refilling the system :-)

As this is a combi boiler, the other alternative is the secondary heat exchanger feeding the hot water system has an internal leak, feeding mains water into the heating circuit. This is about £100 for parts alone so it would be worth considering getting an engineer to fix the boiler - typical one-off call out is about £300 as the implication of screwing anything else up and recommissioning the system properly are a risk. Worcester Bosch may also offer a basic minimum length service contract for about the same money. I know Glowworm do

XXX
December 2016